In recent years, associated with miniaturization and higher density of electronic apparatuses, high integration of semiconductor chips is being under way. Along with this, various proposals have been made on the package forms of semiconductors. For example, as the insulated substrates of packages, use is made of organic package substrates such as those comprised of epoxy resins or polyimide resins and inorganic substrates such as those comprised of silicon as well as metal plates for heat dissipation, and so forth. Therefore, the bonding of such different types of materials has become an important elementary technology on which the reliability of the whole package depends.
On the other hand, in the manufacturing process of packages, there are cases where in order to increase the workability, adhesives are applied in advance onto a surface of one of the substrates to be bonded to each other. Since the adhesives used in such a process are undergo processing steps of the substrate, and are thus required not to alter their characteristics when subjected to treatments with heat or chemicals. Furthermore, the adhesives used around semiconductors also undergo a reflow step; hence it is also important that their reduction in bonding strength is small when being subjected to high temperature, and that they contain less volatile components that cause contamination of the circuitry.
The adhesives that have heretofore been used for these purposes include thermosetting resins such as epoxy resins and acrylic resins and thermoplastic resins such as polyimide resins. Usually, the process of bonding is performed such that an adhesive (layer) is formed on one of the substrates to be adhered, and the substrates are bonded by using heat, pressure or the like. However, in an actual manufacturing process, a substrate on which an adhesive layer has been formed may in some cases undergo a heating step such as drying or preheating prior to its bonding. If the substrate undergoes such a step, then the thermosetting resins as described above will readily cure and their characteristics as adhesives are considerably decreased. If the exposure to the above-mentioned step is to be avoided, the coating of varnish for adhesive or application of an adhesive film must be performed immediately before the bonding. This significantly impairs the freedom of operation process.
On the other hand, thermoplastic resins used as hot-melt type adhesives must have higher glass transition temperature when it is intended to give sufficient heat resistance. To attain this, a very high temperature is required at the time of bonding, so that there is a fear in that substantial thermal damages will occur to the materials around them. Furthermore, when it is attempted to perform bonding at lower temperatures, the glass transition temperature of the thermoplastic resin must be low, which cause such a problem in that the heat resistance of the resin is decreased.
Hence, JP-A-08-034968 etc. also propose a blend type adhesive comprised of a mixture of a thermoplastic resin and a thermosetting resin. Also, there has been a report on a blend of a polyimide resin, which is thermoplastic, and an epoxy resin, which is thermosetting to secure workability and heat resistance. However, the unused components tend to remain and cause an increase in out gas components. Also utilization of siloxane polyimide resins excellent in adhesiveness as thermoplastic adhesives has been proposed. However, the siloxane polyimide resins show a considerable decrease in elastic modulus at the time of heating and cause the problem of peeling off or the like at the time of heating. Accordingly, while it is sure that the blend of a siloxane polyimide resin having an active group and an epoxy resin in order to increase the heat resistance can reduce or prevent a decrease in elastic modulus at the time of heating, but the blend like other thermosetting adhesives raises a problem in that its curing proceeds at relatively low temperatures as low as 200° C. or less, which imposes a significant limit on the production process than ever and extremely impairs the freedom of operation process.
Incidentally, as the method of connecting a semiconductor to a mother board, a transfer sealing method comprising mounting a semiconductor on a lead frame and transfer sealing it with an epoxy resin sealant has been known widely for long time. Recently, however, attention has been focused on a wiring board (interposer) having a polyimide tape as an insulating layer from the viewpoints of using narrower lines, heat release properties, and transmission properties.
Laminates (three-layer tapes) comprised by a polyimide film and a copper foil applied to each other with an adhesive are previously known widely as means of connecting a semiconductor for driving liquid crystals of TAB (tape automated bonding) type connection system. However, it cannot be said that they have sufficient heat resistance, electrolytic corrosion and laser processability for forming connection vias due to an influence of epoxy or acrylic-based adhesives. As a result, their application to packages, for which high reliability is required, has been limited.
Also, there has been a method of forming interposers by using a flexible substrate (two layer tape material) comprised by a polyimide film and a copper foil directly bonded to each other. In this case, too, the problem has arisen that no satisfactory reliability can be obtained because interposers cause blistering at a reflow temperature near 250° C. due to the adhesives having low heat resistance used for bonding and sealing with the semiconductor, or those used for bonding with a heat sink and a stiffener, or due to their insufficient resistance to electrolytic corrosion. Another problem is that even if one of adhesives having high heat resistance was applied, such adhesives, because they are of the thermosetting type, would have a deteriorated flowability in a thermocompression bonding step due to the thermal hysteresis prior to the thermocompression bonding step or deteriorated adhesiveness with semiconductor chips.